In order to reduce the temperature response of SAW filters, they are provided with a compensation layer—typically comprising SiO2. A side effect of this measure, however, is that the coupling is reduced. Broadband filters having such compensation layers can therefore be realized only on highly-coupled substrates.
Bandpass filters made of SAW resonators having a compensation layer can, for example, be made on lithium niobate crystals having a red-128 cut angle. The resonance frequency of the acoustic Rayleigh mode is used on this substrate material.
In many filters with certain material combinations for electrodes and layers deposited thereon, and/or for certain layer thickness combinations, however, a parasitic SH mode (shear horizontal mode) is capable of propagation on lithium niobate. The resonance frequency of the SH mode is above the resonance frequency of the Rayleigh mode. For the serial resonators of a filter, the SH resonances are in the range of the upper passband edge of the filter and cause drops in the transmission function. Even if the geometry of this filter is optimized for maximum suppression of the SH mode, it can be enhanced as a result of tolerance-related geometric deviations and under temperature and power loads. This can cause an increased temperature and power load on the resonators that could result in premature wear and, ultimately, failures of the filter.
Furthermore, today's front-end modules must serve more and more frequency bands, so that the mutual isolation of the various filters is an ever-greater task. In particular, the Tx filters must be designed so that they have sufficient attenuation in parallel Rx bands.